(1) Field of the Invention
This invention relates generally to switching converters and relates more specifically to buck-boost switching regulators having a number N of outputs providing N output signals and at least one inductor and to a method for controlling such a switching converter and to control circuits and methods thereof.
(2) Description of the Prior Art
A switching regulator provides a regulated output voltage V1 to a load based on an unregulated input voltage source Vg. The unregulated input voltage source Vg may be provided by means of a battery.
FIG. 1 illustrates a conventional buck-boost switching regulator power stage that may—depending on the control sequences of the switches—regulate output voltages V1 that are higher, the same or lower than the input voltage Vg.
Hereby, the four switches S1, S2, S3 and S4 control the transfer of the energy from the input node Vg to the output node V1 via an energy storing element (i.e. an inductor).
Depending on the voltage level provided at the input note Vg, there exist different operation modes to generate the required output node voltage V1, depending on if V1 is required to be lower (i.e. V1<Vg) or higher (i.e. V1>Vg) than the input voltage Vg.
Hereby, the mode with V1<Vg is referred to as buck-mode, wherein the mode with V1>Vg is referred to as boost-mode.
A third mode is referred to as buck-boost mode and is established when the required output voltage V1 is close to the input voltage Vg. This buck-boost mode is established to support a smooth transition between the affiliated buck and boost modes.
Assuming the control signals of the four switches S1, S2, S3 and S4 are d1, d2, d3 and d4 (note (for x=1 to 4): Sx is closed (conducting) if dx=1, and Sx is open (non-conducting) if dx=0).
Because S1 and S2, S3 and S4 are complementary switches, there are two independent control signals needed in the power stage. According to the switches states, the power stage can be divided into four states:
State1: d1=1 & d3=1
State2: d1=1 & d3=0
State3: d1=0 & d3=0
State4: d1=0 & d3=1
A duty ratio D0 is in the following defined in relation to a normalized period or a normalized time span of 1.
Buck mode (d1=D0, d3=0)
In the buck mode the VLX2 terminal of the inductor remains always connected to the output terminal V1, that means that S3 is always in non-conducting state (i.e. S3=0) and S4 is always in conducting state (i.e. S4=1).
Switches S1 and S2 are controlled to change their state inverse phased such that for a switching sequence of a normalized period of 1, S1=1 for duty ratio D0, wherein S2=1 for the subsequent normalized time span (1−D0).
The resulting buck-mode control signals and states are depicted in FIG. 2.
Under steady-state input voltage and output load conditions, this results in an output voltage V1, that depends on the input terminal voltage Vg and the duty ratio D0 as follows: V1=Vg*D0/1. The coil current is equivalent to the output current (IL=I1).
Boost mode (d1=1, d3=D0):
In the boost mode the VLX1 terminal of the inductor remains always connected to the input terminal Vg, that means that S2 is always in non-conducting state (i.e. S2=0) and S1 is always in conducting state (i.e. S1=1).
Switches S3 and S4 are controlled to change their state inverse phased such that for a switching sequence of normalized period 1, S3=1 for duty ratio D0, wherein S4=1 for the subsequent normalized time span (1−D0).
Under steady-state input voltage and output load conditions this results in an output voltage V1 that depends on the input terminal voltage Vg and the duty ratio D0 as follows: V1=Vg*1/(1−D0) wherein d1=1, d3=D0
For the averaged inductor and output currents under steady state conditions results:
IL=I1*1/(1−D0). The control sequence is depicted in FIG. 3.
Document EP 1 146 629 A2 describes a control circuit and a method for maintaining high efficiency in a buck-boost switching regulator. The switching regulator can regulate an output voltage higher, lower, or the same as the input voltage. The switching regulator may be synchronous or non-synchronous. The control circuit can operate the switching regulator in buck mode, boost mode, or buck-boost mode. In buck mode, the switching regulator regulates an output voltage that is less than the input voltage. In boost mode, the switching regulator regulates an output voltage that is greater than the input voltage. In buck and boost modes, less than all of the switches are switched ON and OFF to regulate the output voltage to conserve power. In buck-boost mode, all of the switches switch ON and OFF to regulate the output voltage to a value that is greater than, less than, or equal to the input voltage.